Steel sheet coated with Zn-Mg binary coating layer excellent in corrosion resistance and manufacturing method thereof

ABSTRACT

A Zn-Mg binary coating layer formed on a steel sheet has the tri-layered structure that the first sublayer composed of a Zn-Mg alloy having Mg concentration of 0.5 wt. % or less, the second sublayer composed of a Zn-Mg alloy having Mg concentration of 7 wt. % or more and the third sublayer composed of a Zn-Mg alloy having Mg concentration of 0.5 wt. % or less are successively laminated. The coating layer may have the penta-layered structure wherein sublayers composed of a Zn-Mg alloy having Mg concentration of 2-7 wt. % are additionally interposed between the high-Mg and low-Mg sublayers. A Zn-Fe or Zn-Fe-Mg alloy layer may be formed at the boundary between the substrate steel and the coating layer. The adhesion ratio of the first sublayer to the top sublayer is preferably 1.2 or more, while the high-Mg sublayer is preferably conditioned to the mixed structure of a Zn 2  Mg phase with a Mg-dissolved Zn phase. Due to the specified lamellar structure, the intrinsic properties of the high-Mg sublayer is sufficiently exhibited, and the secondary paint adhesiveness of the coating layer is excellent. Consequently, the coated steel sheet is useful as structural members or parts exposed to a severe corrosive atmosphere in various industrial fields.

This is a divisional of application Ser. No. 08/607,703 filed on Feb.27, 1996 now U.S. Pat. No. 5,648,177.

BACKGROUND OF THE INVENTION

The present invention relates to a steel sheet coating with a Zn-Mgbinary coating layer excellent in corrosion resistance, anti-powdering,adhesiveness, spot weldability, anti-discoloring and water-proofsecondary paint adhesiveness. The present invention is also concernedwith a method of manufacturing said Zn-Mg alloy coated steel sheet byvacuum deposition process.

Various finishing methods have been developed so far in order to improvethe corrosion resistance of a steel sheet. A representative method is Znplating. Zn plating is performed by electroplating or hot-dip coating. Ademand for corrosion resistant material having higher properties becomesstronger year by year, in response to the pollution of the atmosphere.In this regard, various improvements have been proposed inelectroplating and hot-dip coating.

In order to improve the corrosion resistance of a Zn-coated steel sheetmanufactured by hot-dip coating process, what is thought at first is theincrease of the adhesion amount of a Zn layer. However, there is alimitation on the increase of the adhesion amount due to operationalconditions, so that corrosion resistance is improved to a limited extentby the increase of the adhesion amount. On the other hand, the increaseof the adhesion amount, i.e. making a plating layer thicker, is likelyto cause defects such as scuffing and flaking during press-working thecoated steel sheet.

In order to form a coating layer with high adhesion amount byelectroplating process, a line speed is necessarily determined lower, orelectrolytic cells are obligatorily increased in number. Consequently,productivity is significantly reduced.

The corrosion resistance can be improved by depositing a Zn alloy suchas a Zn-Ni alloy. However, since the Zn-Ni alloy layer is hard andfragile, defects such as cracking or chipping are likely to be formed inthe coating layer during working the coated steel. When these defectsare formed in the coating layer, the substrate steel is exposed to theatmosphere through the defects. Consequently, the property of thecoating layer itself is not exhibited well, and the defects acts as thestarting points to develop corrosion.

A vapor deposition process is highlighted as the method to overcome theabovementioned problems in the hot-dip coating or electroplatingprocess. Especially, a Zn-Mg alloy-coated steel sheet is expected assuperior corrosion resistant material.

For instance, Japanese Patent Application Laid-Open 64-17853 disclosesthe formation of a Zn-Mg alloy coating layer containing 0.5-40 wt. % Mgand the coating layer mainly composed of Zn-Mg intermetallic compoundseffective in affinity to paint. Japanese Patent Application Laid-Open2-141588 discloses the improvement of the Zn-Mg alloy coating layer inadhesiveness and workability by the formation of an intermediate layersuch as Zn, Ni, Cu, Mg, Al, Fe, Co or Ti between the coating layer andthe substrate steel. Japanese Patent Application Laid-Open 64-25990discloses the provision of a Zn-Ti alloy layer on a Zn-Mg alloy coatinglayer to improve corrosion resistance after painting.

A vapor deposition Zn-coated steel sheet is continuously manufactured byreductively heating a steel sheet in the same unoxidizing and reducingfurnaces as those in a conventional hot-dip process and then vapordepositing one or more coating metals, as disclosed in Nisshin Tech.Review No.56 (1987). p. 41. When the steel sheet is heated in thenonoxidizing furnace, oils remaining on the surface of the steel sheetare burnt and removed from the surface. The steel sheet is then annealedin the reducing furnace held in a gas atmosphere such as H₂ -N₂ or H₂,so that oxide films are decomposed and separated from the surface of thesteel sheet. The steel sheet having the surface activated in this way iscooled in a reducing atmosphere and carried through a duct held in a N₂atmosphere and vacuum sealing means into a vacuum chamber. In the vacuumchamber. Zn is vapor deposited on the steel sheet, and the steel sheetis carried out through outlet vacuum sealing means.

Since the aforementioned process for manufacturing vapor depositionZn-coated steel sheets passes through the same pretreatment step as thatin a conventional hot-dip coating process, a vapor deposition coatingprocess can be performed using a part of existing equipment. Theequipment including the vapor deposition step may be applied to theproduction of a Zn-Mg alloy coated steel sheet excellent in corrosionresistance. A degreasing-cleaning cell may be used instead of theunoxidizing furnace. The vapor deposition Zn coating method has the sameor higher efficiency compared with a Zn electroplating process.

A steel sheet coated with a Zn-Mg alloy layer in big adhesion amount hasthe defect that powdering is likely to form during press working thecoated sheet sheet. Said powdering is accelerated, when the rate ofZn-Mg intermetallic compounds in the coating layer is bigger as theincrease of Mg concentration, or when there are intermetallic compoundsnear the boundary between the coating layer and substrate steel even ifMg concentration is lower. The powdering is caused by hard and fragileZn-Mg intermetallic compounds which can not follow the deformation ofthe substrate steel having high ductility and forms interlayer splittingor cracking in the end.

The powdering may be eliminated by the decrease of Mg concentration forreducing intermetallic compounds in the coating layer and for enhancingthe ductility of the coating layer. However, the decrease of Mgconcentration deteriorates the corrosion inhibiting power of the coatinglayer. Although the powdering can be suppressed by increasing Mgconcentration only at the top layer, the Mg-enriched surface is coloredblack resulting in the reduction of commercial value. Besides, the highMg concentration at the surface of the coating layer accelerates thediffusion of Mg to a welding electrode during spot welding, so that thecoated steel sheet shows poor weldability.

When a Zn-Mg alloy coated steel sheet is manufactured by depositing Mgand then Zn, and heating the deposition layer to promote mutualdiffusion between Mg and Zn, the steel sheet is heated in the reducingatmosphere to remove oxide films from the surface. However, when thesteel sheet having the surface activated is carried through a duct heldin a N₂ atmosphere, the surface is contaminated and re-oxidized by O₂and H₂ O slightly remaining in the N₂ atmosphere. The formed oxide filmsreact with Mg, Fe and Zn and form brittle reaction products.Consequently, the coating layer formed on the substrate steel sometimesshows poor adhesiveness.

When vapor deposition is performed on a steel sheet held at a relativelylower temperature, vacancies are likely to be formed in the coatinglayer. The resultant coating layer is not dense, and the substrate steelis exposed to a corrosive atmosphere through the porous coating layer.Consequently, the coating layer does not exhibit its corrosioninhibiting effect well.

The Zn-Mg alloy coating layer, different from a coating layer formed byconventional hot-dip coating or electroplating process, remarkablychanges its properties in response to its lamellar structure. A steelsheet coated with a Zn-Mg binary coating layer having the laminatedstructure that a high-Mg sublayer formed at the middle is sandwichedwith low-Mg sublayers sometimes shows poor water-proof secondary paintadhesiveness. For instance, the effective adhesiveness of paint film isnot obtained as the result of the test that the coated steel sheet afterbeing painted is dipped in warm water of 50° C. for a long time. Merelythe corrosion resistance can be improved by locating the high-Mgsublayer at the top without the formation of the low-Mg sublayer at thetop. However, the high-Mg sublayer existing at the top of the coatinglayer deteriorates the water-proof secondary paint adhesiveness, butalso promotes discoloration due to dump.

SUMMARY OF THE INVENTION

The present invention is accomplished to overcome the defects asabove-mentioned.

The first object of the present invention is to provide a steel sheetcoated with a Zn-Mg binary coating layer excellent in corrosionresistance, anti-powdering, adhesiveness, spot weldability andanti-discoloring by reforming the coating layer to a multi-layeredstructure having specified composition while suppressing defectsoriginated in high Mg concentration.

The second object of the present invention is to provide a steel sheetcoated with a Zn-Mg binary coating layer well balanced betweenwater-proof secondary paint adhesiveness and corrosion resistance bycontrolling the adhesion amount of a low-Mg sublayer to be formed at thetop.

The third object of the present invention is to provide a steel sheetcoated with a Zn-Mg binary coating layer excellent in corrosionresistance which effectively suppresses the formation of rust even atcut end face by conditioning the high-Mg sublayer to specified mixedstructure.

The fourth object of the present invention is to provide a steel sheetcoated with a dense Zn-Mg binary coating layer excellent in corrosionresistance and adhesiveness by vapor depositing Zn and then Mg.

A steel sheet coated with a Zn-Mg binary coating layer according to thepresent invention has the basic structure that the first sublayercomposed of a Zn-Mg alloy containing 0.5 wt. % or less Mg, the secondsublayer composed of a Zn-Mg alloy containing 7 wt. % or more Mg, andthe third sublayer composed of a Zn-Mg alloy containing 0.5 wt. % orless Mg are successively laminated on the surface of substrate steel. Inthe case when much higher corrosion resistance is requested, the coatinglayer may be adjusted to the penta-layered structure instead of saidtri-layered structure. The penta-layered structure has additionalsublayers composed of a Zn-Mg alloy containing 2-7 wt. % Mg and locatedbetween the high-Mg sublayer and the low-Mg sublayers in the tri-layeredstructure.

Mg concentration in the high-Mg sublayer is preferably up to 20 wt. %,in order to assure corrosion resistance in a humid atmosphere. Thecorrosion resistance of the coated steel sheet is effectively enhanced,when the high-Mg sublayer has the mixed structure of a Zn₂ Mg phase witha Mg-dissolved Zn phase. The middle-Mg sublayer may be conditioned tothe mixed structure of a Zn₁₁ Mg₂ phase with a Mg-dissolved Zn phase.When the top sublayer composed of the low-Mg alloy is formed in anadhesion amount of 0.3 g/m² or more, the coated steel sheet is improvedin water-proof secondary paint adhesiveness. The high-Mg sublayereffective in the improvement of corrosion resistance may be located at ahigher position in the coating layer, by controlling the adhesion ratioof the first sublayer to the top sublayer above 1.2.

When vapor deposition is performed in an atmosphere containing O₂ or H₂O, the surface of a steel sheet would be oxidized. When vapor depositionis not performed immediately after the activation of the steel sheet,the surface of the steel sheet would be contaminated. Such oxidation orcontamination reduces the adhesiveness of a coating layer onto thesubstrate steel. In these cases, a Zn-Fe or Zn-Fe-Mg alloy layer isformed at the boundary between the coating layer and the substrate steelin order to improve the adhesiveness of the coating layer. The Zn-Fe orZn-Fe-Mg alloy layer is preferably of 0.5 μm or less in thickness toinhibit the powdering of the coating layer during working. Feconcentration in the Zn-Fe or Zn-Fe-Mg alloy layer is preferably 6 wt. %or more.

The steel sheet coated with a Zn-Mg binary coating layer is preferablymanufactured by predepositing Zn under the condition that the adhesionratio of Zn to Mg is 1.5 or more per unit surface area, and thendepositing Mg and Zn. The steel sheet is preferably held at atemperature above 180° C. during the deposition of Zn and Mg. The Zn-Mgbinary coating layer having the specified lamellar structure may beobtained by simultaneously depositing Zn and Mg while changing thedeposition ratio of Zn to Mg.

The temperature of a steel sheet at the completion of vapor depositionis an important factor to promote mutual diffusion between deposited Znand Mg by a heat retained in the steel sheet. Said temperature can becontrolled by adjusting the temperature of the steel sheet before vapordeposition. When Zn, Mg and then Zn are independently and successivelyvapor deposited on the steel sheet temperature-controlled so as to beheld at 270°-370° C. at the completion of vapor deposition, theeffective multi-layered structure and the specified mixed structure areformed. A Zn-Mg binary coating layer having the specified structure maybe formed by holding the steel sheet 1 hr. or longer at 150°-250° C.after the successive deposition of Zn, Mg and Zn. The heating in thiscase is performed in an inert gas atmosphere such as N₂ or Ar in orderto inhibit the oxidation of the steel sheet.

The steel sheet to be coated is carried through the reductively heatingzone to activate the surface of the steel sheet, a duct held in a N₂atmosphere and vacuum sealing means into a vacuum chamber. In the vacuumchamber, Zn, Mg and then Zn are successively vapor deposited on thesteel sheet, and the deposited Zn and Mg are mutually diffused to form aZn-Mg binary coating layer. In this case, the steel sheet is preferablypassed through the duct under the condition satisfying the relationshipsof X×Z≦1.2 and Y×Z≦35, wherein X represents O₂ concentration (vol. %) inthe duct, Y represents H₂ O concentration (vol. %) in the duct, and Zrepresents the passing time (sec.) of the steel sheet through the duct.

H₂ may be added in an amount of 0.05-4 vol. % to the atmosphere in theduct. In this case, the steel sheet may be passed through the duct underthe relaxed condition satisfying the relationships of X×Z≦3.8 andY×Z≦80.

There is ordinarily an oxidized Mg-enriched layer on the surface of theZn-Mg binary coating layer formed by the vapor deposition process. TheMg-enriched layer is left as such for the use requiring initialcorrosion resistance. The Mg-enriched layer is removed by acid picklingor the like for the use requiring spot weldability or detesting theblackening of the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the sectional view of a steel sheet coated with a Zn-Mg binarycoating layer having tri-layered structure according to the presentinvention.

FIG. 2 is the sectional view of a steel sheet coated with a Zn-Mg binarycoating layer having penta-layered structure according to the presentinvention.

FIG. 3 is the sectional view of a steel sheet coated with a Zn-Mg binarycoating layer having tri-layered structure having a Zn-Fe or Zn-Fe-Mgalloy layer formed at the boundary.

FIG. 4 is the sectional view of a steel sheet coated with a Zn-Mg binarycoating layer having penta-layered structure having a Zn-Fe or Zn-Fe-Mgalloy layer formed at the boundary.

FIG. 5 is a schematic view illustrating the mixed structure of a high-Mgsublayer.

FIG. 6 is a schematic view illustrating a plant for manufacturing aZn-Mg alloy coated steel sheet.

FIG. 7 is a flow chart illustrating a complexed cycle corrosion test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We have researched and examined the effect of the lamellar structure ofa Zn-Mg binary coating layer on the properties of the coated steel sheetfrom various points of view. In the course of researching, we have foundthat the coated steel sheet is improved in anti-powdering, corrosionresistance, spot weldability, adhesiveness and anti-discoloring byforming a high-Mg sublayer at the middle of the coating layer andsandwiching said high-Mg sublayer between low-Mg sublayers.

The Zn-Mg binary coating layer according to the present invention hasthe tri-layered structure (as shown in FIG. 1) that a low-Mg sublayer asthe first sublayer, a high-Mg sublayer as the second sublayer andanother low-Mg sublayer as the third sublayer are successively laminatedon the surface of substrate steel. The low-Mg sublayer having Mgconcentration of 0.5 wt. % or less has relatively higher dissolutionspeed and serves as a sacrifice anode effective for inhibiting theformation of rust at the exposed surface part of the substrate steelsuch as scratched parts. Said low-Mg sublayer especially inhibits theformation of rust at the initial period. The high-Mg sublayer having Mgconcentration of 7 wt. % or more (preferably 7-20 wt. %) has excellentcorrosion preventing effect and prolongs the life of the coating layeritself until consumption by corrosion reaction. The element Mg dissolvedfrom the high-Mg sublayer promotes the formation of corrosion productssuch as ZnCl₂.4 Zn(OH)₂ and Zn(OH)₂ serving as corrosion inhibitors, soas to improve corrosion resistance.

The coating layer may have the penta-layered structure (as shown in FIG.2) that middle-Mg sublayers having Mg concentration of 2-7 wt. % areformed between the high-Mg sublayer and the low-Mg sublayers, sublayers.The middle-Mg sublayer has intermediate properties between the high-Mgand low-Mg sublayers, and further improves the corrosion resistance.Consequently, the coated steel sheet exhibits the excellent corrosionresistance which has not been noted in known coated steels, due to thesynergetic effects of the sublayers above-mentioned.

The high-Mg sublayer significantly reduces the adhesiveness of a paintfilm, when water or aqueous vapor invades to the boundary between thecoating layer and the paint film. The water-proof secondary paintadhesiveness which is reduced by the high-Mg sublayer is recovered bythe formation of the low-Mg sublayer having Mg concentration of 0.5 wt.% or less in an adhesion amount of 0.3 g/m² or more on the high-Mgsublayer.

If the top low-Mg sublayer is formed in an adhesion amount below 0.3g/m², the parts of the high-Mg sublayer uncovered with the low-Mgsublayer remain as pin holes exposed to the atmosphere. The exposedparts of the high-Mg sublayer puts harmful influence on water-proofsecondary paint adhesiveness. In addition, the high-Mg sublayer forms athicker reaction layer than the low-Mg sublayer does, during chemicalconversion or painting. The thicker reaction layer reduces thewater-proof secondary paint adhesiveness, too. In this regard, thelow-Mg sublayer to be formed at the top shall have Mg concentration of0.5 wt. % or less and an adhesion amount of 0.3 g/m² or more, to improvewater-proof secondary paint adhesiveness.

The corrosion preventing effect of the coating layer is guaranteed bycontolling the adhesion ratio of the first low-Mg sublayer to the toplow-Mg sublayer at 1.2 or more. The adhesion ratio of 1.2 or more meansthe location of the high-Mg sublayer effective in corrosion resistanceat a higher position in the coating layer. If the adhesion ratio isbelow 1.2, the improvement of corrosion resistance is not notedregardless of the position of the high-Mg sublayer. Since the low-Mgsublayer having Mg concentration of 0.5 wt. % or less is ductile, itabsorbs the difference in deformation between the high-Mg coating layerand the substrate steel during press working the coated steel sheet.Consequently, the low-Mg sublayer effectively inhibits powdering, too.

When the high-Mg sublayer having Mg concentration of 7-20 wt. % isconditioned to the mixed structure (as shown in FIG. 5) of a Zn₂ Mgphase with a Mg-dissolved Zn phase, excellent corrosion resistance isexhibited even at the sectional part of the coated steel, e.g. a cut endface, exposed to the outside. The effect of the mixed structure on theimprovement of corrosion resistance is supposed as follows:

When the Zn-Mg alloy coated steel sheet is exposed to a corrosiveatmosphere, the element Mg dissolves from the Zn₂ Mg phase which is at abasic potential by the electrochemical reaction between a Mg-dissolvedZn phase and the Zn₂ Mg phase. The dissolved Mg reacts with water andforms hydroxide effective in corrosion prevention. Since the cut endface is covered with the hydroxide, the cut end face is protectedagainst corrosion. The effect of the controlled metallurgical structureof the coating layer on corrosion resistance, which is newly found outby the present invention, is the reason why the excellent corrosionresistance has not been noted in conventional coated steel sheets. Onthe contrary, a coating layer comprising intermetallic compounds such asZn₁₁ Mg₂, Zn₂ Mg and ZnMg, as disclosed in Japanese Patent ApplicationLaid-Open 1-139755, does not generate big electric potential differencebetween Zn₁₁ Mg₂ and Zn₂ Mg. Consequently, electrochemical reaction isweak, the dissolution of Mg is dull, and the effect on corrosionprevention at the cut end face is inferior.

When the middle-Mg sublayer having Mg concentration of 2-7 wt. % to beformed as the second and fourth sublayers (shown in FIG. 2) isconditioned to the mixed structure of a Zn₁₁ Mg₂ phase with aMg-dissolved Zn phase, it is possible to continue the dissolution of Mgfor a long time. In this case, since the Zn₁₁ Mg₂ phase has lower Mgconcentration than the Zn₂ Mg phase, electric potential differencegenerated between the Mg-dissolved Zn phase and the Zn₁₁ Mg₂ phase is sosmall to lower the dissolution speed of Mg. Consequently, thedissolution of Mg is gradually continued, so as to assure the excellentcorrosion inhibiting effect of the coating layer for a long period.

In short, the cut end face at the initial stage, which requires bigcorrosion inhibiting power due to the naked state, is prevented fromcorrosion by the high-Mg sublayer (i.e. the third sublayer shown in FIG.2) having high dissolution speed of Mg and covered with magnesiumhydroxide as a reaction product. After the face is covered withmagnesium hydroxide, so big corrosion inhibiting power is not necessary.The corrosion inhibiting power of the second and fourth sublayers (shownin FIG. 2) is applied to the covered face. Therefore, the cut end faceis protected from corrosion over a long time.

A Zn-Fe or Zn-Fe-Mg alloy layer is preferably formed at the boundarybetween the substrate steel and the first sublayer, as shown in FIGS. 3and 4, to improve the adhesiveness of the coating layer. When the Zn-Mgbinary coating layer is formed by vacuum deposition process in anatmosphere containing O₂ or H₂ O, the adhesiveness of the coating layeris deteriorated due to the surface oxidation of the steel sheet. Whenvapor deposition is not performed immediately after surface activation,the adhesiveness of the coating layer is deteriorated by thecontamination of the surface, too. Such the deterioration ofadhesiveness is inhibited by the formation of the Zn-Fe or Zn-Fe-Mgalloy layer at the boundary. The Zn-Fe or Zn-Fe-Mg alloy layerpreferably of 0.5 μm or less in thickness: otherwise powdering would beformed during working the coated steel sheet. Fe concentration in theZn-Fe or Zn-Fe-Mg alloy layer is preferably 6 wt. % or more.

When a steel sheet after being coated is acid pickled to remove anoxidized Mg-enriched layer, the surface of the coating layer which willcome in contact with a welding electrode during spot welding isconditioned to a low-Mg state having Mg concentration of 0.5 wt. % orless. The low-Mg state reduces the diffusion of Mg to the weldingelectrode and effectively improves spot weldability. If Mg remains in alarge amount on the surface of the coating layer, the surface of thecoating layer would be colored black due to the state unsaturated withzinc oxide and hydroxide. The blackening is inhibited by controlling Mgconcentration on the surface at 0.5 wt. % or less, too.

According to the present invention, Zn is vapor predeposited on thesteel sheet before Mg deposition. The predeposited Zn firmly adheresonto the surface of the steel sheet even which has thin oxide filmsthereon, so that Mg and Zn depostion layers in the following steps areformed on the substrate steel with good adhesiveness. However, when ittakes a few seconds or longer from Mg deposition to Zn deposition, orwhen vapor deposition is performed on a steel sheet held at atemperature of 180° C. or higher to improve the adhesiveness of thecoating layer, too thin primary Zn deposition permits the diffusion ofMg in a large amount to the boundary between the coating layer and thesubstrate steel and causes the formation of a brittle intermediatelayer. The similar brittle intermediate layer would be formed by thediffusion of Mg to the Zn predeposition layer during the heat treatmentof the coating layer.

The formation of the brittle intermediate layer is inhibited bycontrolling the adhesion ratio of the primary Zn deposition layer to theMg deposition layer at 1.5 or more. The primary Zn deposition layer inthe controlled adhesion amount suppresses the diffusion of Mg to theboundary between the coating layer and the substrate steel, whendeposition is performed on the steel sheet held at 180° C. or higher orwhen the coating layer is subjected to thermal diffusion treatment. Evenif Mg is diffused, the diffusion of Mg is controlled at a lower level.Consequently, the brittle intermediate layer is not formed, and theadhesiveness of the Zn-Mg binary coating layer to the substrate steel isenhanced.

When the temperature of the steel sheet is adjusted at 270°-370° C. atthe completion of vapor deposition, the diffusion reaction between Znand Mg is autogeneously promoted by a heat retained in the steel sheet,so as to form the Zn-Mg binary coating layer having the tri-layeredstructure (shown in FIG. 1) or the penta-layered structure (shown inFIG. 2). If the temperature of the steel sheet does not reach 270° C. Mgpartially remains as such without diffusion. The remaining Mg causespoor corrosion resistance. If the temperature of the steel sheet exceeds370° C. the coating layer is changed to the state that Zn-Mgintermetallic compounds and a Mg-dissolved Zn phase are dispersed overthe whole of the coating layer, resulting in poor paint adhesiveness andthe blackening of the coating layer. The higher temperature of the steelsheet above 370° C. causes the excessive growth of the Zn-Fe or Zn-Fe-Mgalloy layer exceeding 0.5 μm in thickness. Due to the thick Zn-Fe orZn-Fe-Mg alloy layer, cracking or peeling would be formed in the coatinglayer during press working the coated steel sheet. The heatingtemperature is preferably determined at 150°-240° C. by the same reason,when the coating layer is heat treated for the diffusion of Zn and Mgafter the completion of vapor deposition.

When a steel sheet to be coated is passed through a duct held in a N₂atmosphere, oxide films which are formed on the surface of the steelsheet by re-oxidation is prevented from thickening by satisfying theconditions of X×Z≦1.2 and Y×Z≦35, wherein X represents O₂ concentration(vol. %) in the N₂ atmosphere, Y represents H₂ O concentration (vol. %)in the N₂ atmosphere, and Z represents the passing time of the steelsheet through the duct. Consequently, the adhesiveness of the primary Zndeposition layer onto the substrate steel is assured. When H₂ in a smallamount is added to the N₂ atmosphere, the restrictions on the atmospherein the duct and the passing time are relaxed so as to facilitate theproduction of the coated steel sheet.

The coated steel sheet according to the present invention ismanufactured by a plant schematically shown in FIG. 6.

A parent steel sheet 1 is uncoiled from a pay-off reel 2, carriedthrough a pretreating zone 10 comprising an unoxidizing furnace 11, areductively annealing furnace 12 and a cooling zone 13 held in areducing atmosphere, and then introduced through a chamber 19 whoseatmosphere is replaced by N₂ and a duct 20 held in a N₂ atmosphere intoa vacuum vapor deposition chamber 30. In the reductively annealingfurnace 12, the parent sheet 1 is heated and annealed in a reducingatmosphere having the composition of 50% H₂ -N₂ to remove oxide filmsfrom the surface.

The vacuum chamber 30 is hermetically sealed by inlet sealing rolls 31and outlet sealing rolls 32. The interior of the vacuum chamber 30 isevacuated to the degree of vacuum of 1×10⁻² torr. by a vacuum pump (notshown). A primary Zn deposition compartment 40, a Mg depositioncompartment 50, the first Zn deposition compartment 60 and the second Zndeposition compartment 70 are sequentially arranged along the pass lineof the parent steel sheet 1 in the vacuum chamber 30. A subsidiary Zndeposition compartment 65 may be occasionally located between the firstZn deposition compartment 60 and the second Zn deposition compartment70.

Vapor Mg deposition may be performed using a heat source such aselectric resistance heating, high-frequency heating, electron beamheating or arc heating. In the illustrated plant, Mg evaporation sources51 and guide hoods 52 for introducing Mg vapor are located at thepositions facing to both surfaces of the parent sheet 1. Either one orboth of the Mg evaporation sources 51 are operated in response to singleor double-face coating.

The Zn deposition compartments 40, 60, 70 have Zn vapor generators 41,61, 71 and guide hoods 42, 62, 72 faced to the parent steel sheet 1 orthe steel sheet 3 to which Mg has been vapor deposited. The guide hoods42 in the primary Zn deposition compartment 40 are located at thepositions faced to the both surfaces of the parent steel sheet 1, so asto form primary Zn deposition layers on both surfaces of the steel sheetat the same time. In the first Zn deposition compartment 60 or thesecond Zn deposition compartment, the steel sheet 3 is wound around awinding roll 63 or 73, and the single surface of the steel sheet 3 issubjected to vapor deposition.

The steel sheet 4 after being coated with Zn vapor deposition layer iscarried through the outlet sealing rolls 32 into a heating furnace 80.The coated steel sheet 4 is occasionally heat treated, by proper heatingmeans such as a high-frequency heater in a heating furnace 80. Theheated steel sheet 5 is passed through a post treatment zone 90, whereinchemical conversion treatment or the like is applied to the steel sheet5. The steel sheet is finally coiled as a coated steel sheet 6 around acoiling reel 7. When the steel sheet is not heat treated in the heatingfurnace 80, the coated steel sheet 4 may be batch heated in anindependent heating furnace.

The coated steel sheet manufactured in this way has a Zn-Mg binarycoating layer or layers on either one or both surfaces according to thedemands. For instance, when the Zn-Mg binary coating layer is formed onthe single surface of the steel sheet, one of the Mg evaporation sources51 and one of the Zn vapor generators 61 or 71 are driven.

EXAMPLES Example 1

Steps of Manufacturing Coated Steel Sheet

A steel sheet of 0.8 mm in thickness having the composition of 0.002 wt.% C, 0.02 wt. % Si, 0.21 wt. % Mn, 0.007 wt. % P, 0.001 wt. % S, 0.076wt. % Ti. 0.031 wt. % Al and the balance being Fe was used as a parentsheet. This parent sheet was reductively heated in the gas atmosphere ofN₂ -50% H₂ to remove oxide films from the surface, and then fed into thevacuum chamber which had been evacuated by a vacuum pump and kept at theN₂ partial pressure of 5×10⁻² torr, by supplying N₂ gas having a dewpoint of -60° C.

In the vacuum chamber, the vapor deposition was performed in the orderof Zn→Mg→Zn under the conditions that an adhesion amount in total waspredetermined at 100 g/m² and that the deposition of primary Zn wascontrolled at the same adhesion amount as that of the secondary Zndeposition.

In the case of manufacturing a coated steel sheet having a Zn-Fe orZn-Fe-Mg alloy layer formed between substrate steel and the coatinglayer having the tri-layered structure (shown in FIG. 3) or thepenta-layered structure (shown in FIG. 4), the vapor deposition in theorder of Zn→Mg→Zn was performed on the parent sheet held at 200° C., theparent sheet was heated 5-10 seconds in the vacuum chamber filled withN₂ gas at 700 torr. The heating temperature was determined at 270°-300°C. for forming the tri-layered structure, or at 330°-370° C. for formingthe penta-layered structure. The Zn-Fe or Zn-Fe-Mg alloy layer grew upto approximately 0.2 μm in thickness by the heat treatment. The obtainedcoating layer had the multi-layered structure that a high-Mg sublayernear the middle part of the coating layer had Mg concentration ofapproximately 10 wt. %. while low-Mg sublayers on and under said high-Mgsublayer had Mg concentration of 0.5 wt. % or less. Middle-Mg sublayersin the penta-layered structure had Mg concentration of approximately 4wt. %.

On the other hand, Zn and Mg were vapor deposited while variouslychanging their deposition ratio, so as to form Zn-Mg binary coatinglayers having the multi-layered structure shown in FIGS. 1 and 2 with anadhesion amount of 100 g/m² per single face. In this case, theconcentration of each sublayer was adjusted to the same value as thesublayers afore-mentioned (shown in FIGS. 3 and 4). The parent sheet washeld at 120° C. during vapor deposition, but not heat treated after thevapor deposition.

Each coated steel sheet was acid pickled in a 0.5% HCl aqueous solutionto remove Mg-rich layers from the surface. The coated steel sheet afterthe acid pickling was sufficiently washed with water. When the coatedsteel sheets obtained in this were observed, the coating layers havingmulti-layered structures shown in Table 1 were detected.

Research for Properties of Coating Layers

Each coated steel sheet was examined to research the properties of thecoating layer. Corrosion resistance was evaluated by a time period untilthe formation of rust in the salt water spray test regulated under JISZ2371. Anti-powdering was evaluated by the degree of powdering in thedraw bead test wherein a test piece was clamped with dies to which beadsof 4 mm in height with a radius of 0.5 mm were formed, and the testpiece was drawn from the dies at a drawing speed of 200 m/min. with apressure of 500 kgf. Spot weldability was evaluated by the number ofspots capable of continuous welding under the condition using a type-CFelectrode made of a Cu-1% Cr alloy and having a top of 4.5 mm indiameter attached to a single-phase A.C. welder. Anti-blackening wasevaluated by the difference ΔL* of brightness between before and afterthe test wherein a testpiece was held alone 1000 hrs. in an accelerateddiscoloring device at 50° C. and relative humidity of 60%.

It is noted from Table 1 that each steel sheet coated with the coatinglayer having the tri-layered or penta-layered structure according to thepresent invention is excellent in all of corrosion resistance,anti-powdering, spot weldability and anti-blackening. In addition, thecoating layer having the Zn-Fe or Zn-Fe-Mg layer at the boundary (asshown in FIG. 3 or 4) exhibited excellent adhesiveness, even when vapordeposition was performed in an oxidizing atmosphere containing H₂ O orO₂ in an amount of several tens p.p.m.

                                      TABLE 1                                     __________________________________________________________________________    PROPERTIES OF COATING LAYERS IN RELATION WITH                                 MULTI-LAYERED STRUCTURE (THE PRESENT INVENTION)                                              TRI-LAYERED PENTA-LAYERED                                                                          TRI-LAYERED                               MULTI-LAYERED STRUCTURE                                                                      STRUCTURE   STRUCTURE                                                                              STRUCTURE                                 OF COATING LAYER                                                                             (FIG. 3)    (FIG. 4) (FIG. 1)                                  __________________________________________________________________________    av. Mg conc. (wt. %)                                                                         3   5   7   7        7                                         time (hrs.) until                                                                            770 890 1010                                                                              1100     1010                                      formation of rust                                                             powdering (g/m.sup.2)                                                                        0.20                                                                              0.29                                                                              0.37                                                                              0.46     0.25                                      number of spots capable                                                                      1150                                                                              1200                                                                              1200                                                                              1200     1200                                      of continuous welding                                                         difference (ΔL*) in.                                                                   -3  -3  -4  -3       -3                                        brightness representing                                                       blackening                                                                    __________________________________________________________________________

Comparative Example 1

The same parent sheet was heated at 120° C., and a Zn-Mg alloy coatinglayer having the homogeneous composition was formed with an adhesionamount of 100 g/m² per single face by the simultaneous deposition of Znand Mg.

Comparative Example 2

Vapor deposition in the order of Zn→Mg was performed on the same parentsheet held at 200° C., and then the parent sheet was heated at 270°-330°C. The coating layer formed in this way had the bi-layered structurecomprising an under sublayer composed of a Zn-Mg alloy containing 0.5wt. % or less Mg and an upper sublayer composed of a Zn-Mg alloycontaining 10 wt. % or more Mg. In this case, an adhesion amount wascontrolled at 100 g/m² per single face (corresponding to the thicknessof approximately 0.2 μm), and the temperature of the parent sheet wascontrolled so as to form a Zn-Fe or Zn-Fe-Mg alloy layer at the boundarybetween the substrate steel and the coating layer.

Comparative Example 3

Vapor deposition in the order of Mg→Zn was performed on the same steelsheet with an adhesion amount of 100 g/m² per single face, and the steelsheet was heated at 270°-330° C. so as to form a Zn-Mg alloy coatinglayer having the bi-layered structure comprising an upper sublayerhaving Mg concentration of approximately 10 wt. % or less and an uppersublayer having Mg concentration of 0.5 wt. % or more.

Comparative Example 4

The same steel sheet held at 90° C. was subjected to vapor deposition inthe order of Zn deposition→simultaneous Zn and Mg depositions→Zndeposition. The coating layer formed in this way had the tri-layeredstructure comprising an under Zn sublayer, a middle sublayer composed ofa Zn-Mg alloy having Mg concentration of approximately 10 wt. % and anupper Zn sublayer. In this case, an adhesion amount was adjusted to 100g/m² per single face.

Each coated steel sheet was acid pickled 10 minutes in a 0.5%-HClsolution to remove Mg-rich layers from the surface and then sufficientlywashed with water. The coated steel sheets were examined by the same wayto research the properties of the coating layers. The results are shownin Table 2.

                                      TABLE 2                                     __________________________________________________________________________    PROPERTIES OF COATING LAYERS IN RELATION WITH                                 MULTI-LAYERED STRUCTURE (COMPARATIVE EXAMPLE)                                 NO. OF COMPARATIVE                                                            EXAMPLE     1    2    3       4                                               STRUCTURE OF                                                                              HOMOGENEOUS                                                                             BI-LAYERED                                                                            TRI-LAYERED                                     COATING LAYER                                                                             Mg. CONC. STRUCTURE                                                                             STRUCTURE                                       __________________________________________________________________________    av. Mg conc. (wt. %)                                                                      5    7    7   7   7                                               time (hrs.) until                                                                         790  890  1010                                                                              940 670                                             formation of rust                                                             powdering (g/m.sup.2)                                                                     10.8 19.3 0.21                                                                              3.35                                                                              0.15                                            number of spots capable                                                                   600  550  400 1200                                                                              1200                                            of continuous welding                                                         difference (ΔL*) in                                                                 -15  -19  -25 -3  -3                                              brightness representing                                                       blackening                                                                    __________________________________________________________________________

It is noted from Table 2 that the coated steel sheet in any of thecomparative examples was inferior in corrosion resistance,anti-powdering, spot weldability and anti-blackening. Especially, thecoated steel sheets in the comparative examples 1 and 2 having high Mgconcentration even at the surface showed big difference (ΔL*) inbrightness and did not keep good external appearance. The coated steelsheet in the comparative example 3 caused too much powdering and wasinferior in workability, although Mg concentration was lower at thesurface.

Example 2

Steps of Manufacturing Coated Steel Sheet

An Al-killed steel sheet of 0.7 mm in thickness having the compositionof 0.003 wt. % C, 0.03 wt. % Si, 0.29 wt. % Mn, 0.009 wt. % P, 0.002 wt.% S, 0.03 wt. % Ti, 0.035 wt. % Al and the balance Fe was used as aparent sheet. After this parent sheet was reductively heated in the sameway as that in Example 1, it was carried into the vacuum chamber held ina N₂ atmosphere at the degree of vacuum 5×10⁻² torr.

In the vacuum chamber, vapor deposition in the order of Zn→Mg→Zn wasperformed with an adhesion amount of 20 g/m² per single face. After thevapor deposition, the steel sheet was heated at 270°-350° C. The coatedsteel sheet obtained in this way had a coating layer whose Mgconcentration was 5 wt. % in average. The coating layer had themulti-layered structure shown in FIG. 3 or 4, and a Zn-Fe or Zn-Fe-Mgalloy layer of 0.1-0.3 μm in thickness was formed at the boundarybetween the substrate steel and the coating layer. The highest-Mgsublayer had Mg concentration of approximately 11 wt. %, while thesublayers on and under said highest-Mg sublayer had Mg concentration ofapproximately 0.1 wt. %. The sublayers between the high-Mg and thelow-Mg sublayers had Mg concentration of approximately 4 wt. %.

Each steel sheet coated with the Zn-Mg binary coating layer was acidpickled in a 0.5%-HCl solution to remove Mg-rich layers from thesurface.

The coated steel sheets after the acid pickling was examined to researchsecondary paint adhesiveness and corrosion resistance.

Research for Secondary Paint Adhesiveness

After each coated steel sheet was pretreated by chromating orphosphating, a type-acryl electrodeposition coating of 20 μm inthickness as dried state was applied to the sheet. The painted steelsheet was dipped 1000 hrs. in distilled water at 50° C. and scratchedwith a cross cut pattern in the intervals of 1 mm by a cutter knife. Anadhesive tape was applied onto the scratched surface and then peeledoff. The parts of the coating layer separated from the surface of thetest piece in the state stuck onto the adhesive tape were counted innumber. The secondary paint adhesiveness was evaluated by the number ofseparated parts according to the standards shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        STANDARDS FOR EVALUIATION OF SECONDARY PAINT                                  ADHESIVENESS                                                                  REMARKS RATIO OF SEPARATION BY SURFACE AREA                                   ______________________________________                                        ◯                                                                          ≦5%                                                           Δ 5-50%                                                                 × >50%                                                                  ______________________________________                                    

Research for Corrosion Resistance

Corrosion resistance was evaluated by a time period until the formationof rust in the salt water spray test regulated in JIS Z2371.

The effects of the adhesion amount of the top Zn-Mg alloy sublayerhaving Mg concentration of 5 wt. % or less in the coating layers havingthe tri-layered structure (shown in FIG. 3) and the penta-layeredstructure (shown in FIG. 4) on water-proof secondary paint adhesivenessare shown in Tables 4 and 5, respectively. It is noted from these Tablesthat sufficinet adhesiveness was obtained in the coating layer havingany multi-layered structure, when the top sublayer was formed with anadhesion amount of 0.3 g/m² or more.

                  TABLE 4                                                         ______________________________________                                        SECONDARY PAINT ADHESIVENESS OF Zn--Mg COATING                                LAYER HAVING TRI-LAYERED STRUCTURE SHOWN IN FIG. 3                            ADHESION AMOUNT                                                               OF TOP SUBLAYER                                                                            PRETREATMENT  SECONDARY PAINT                                    (g/m.sup.2)  FOR PAINTING  ADHESIVENESS                                       ______________________________________                                         0.06        A             ×                                            0.1          A             Δ                                            0.3          A             ◯                                      0.6          A             ◯                                                   B             ◯                                      1.1          A             ◯                                                   B             ◯                                      2.0          A             ◯                                                   B             ◯                                      3.3          A             ◯                                                   B             ◯                                      4.6          A             ◯                                                   B             ◯                                      ______________________________________                                         NOTE: A is chromating, and B is phosphating.                             

                  TABLE 5                                                         ______________________________________                                        SECONDARY PAINT ADHESIVENESS OF Zn--Mg COATING                                LAYER HAVING PENTA-LAYERED STRUCTURE                                          SHOWN IN FIG. 4                                                               ADHESION AMOUNT                                                               OF TOP SUBLAYER                                                                            PRETREATMENT  SECONDARY PAINT                                    (g/m.sup.2)  FOR PAINTING  ADHESIVENESS                                       ______________________________________                                         0.05        A             ×                                            0.1          A             Δ                                            0.3          A             ◯                                      0.7          A             ◯                                                   B             ◯                                      1.3          A             ◯                                                   B             ◯                                      2.0          A             ◯                                                   B             ◯                                      ______________________________________                                         NOTE: A is chromating, and B is phosphating.                             

The effects of the adhesion ratio of the first sublayer to the topsublayer on corrosion resistance are shown in Tables 6 and 7. In thiscase, the first and top sublayers were composed of a Zn-Mg alloycontaining 0.5 wt. % or less Mg, and the top sublayer was formed with anadhesion amount of 0.3 g/m² or more. It is noted from these Tables thatthe time period until the formation of rust was prolonged in any coatinglayer having the tri-layered or penta-layered structure by controllingthe adhesion ratio at 1.2 or more, resulting in good corrosionresistance.

                  TABLE 6                                                         ______________________________________                                        THE EFFECT OF ADHESION RATIO OF FIRST SUBLAYER TO                             TOP SUBLAYER ON CORROSION RESISTANCE IN COATING                               LAYER HAVING TRI-LAYERED STRUCTURE SHOWN IN FIG. 3                            ADHESION RATIO OF                                                                           TIME (HRS.) UNTIL FORMATION OF                                  FIRST/TOP SUBLAYER                                                                          5% RUST IN SALT WATER SPRAY TEST                                ______________________________________                                        0             120                                                             0.5           120                                                             1.0           120                                                             1.2           132                                                             1.9           148                                                             3.3           156                                                             4.1           156                                                             ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        THE EFFECT OF ADHESION RATIO OF FIRST SUBLAYER TO                             TOP SUBLAYER ON CORROSION RESISTANCE IN COATING                               LAYER HAVING PENTA-LAYERED STRUCTURE SHOWN                                    IN FIG. 4                                                                     ADHESION RATIO OF                                                                           TIME (HRS.) UNTIL FORMATION OF                                  FIRST/TOP SUBLAYER                                                                          5% RUST IN SALT WATER SPRAY TEST                                ______________________________________                                        0             132                                                             0.4           132                                                             0.9           132                                                             1.2           144                                                             1.8           150                                                             2.6           162                                                             ______________________________________                                    

Example 3

Steps of Manufacturing Coated Steel Sheet

A steel sheet of 0.5 mm in thickness having the composition of 0.005 wt.% C. 0.04 wt. % Si, 0.33 wt. % Mn, 0.008 wt. % P, 0.003 wt. % S, 0.04wt. % Ti, 0.046 wt. % Al and the balance Fe was used as a parent sheet.This parent sheet was reductively heated by the same way as that inExample 1 and then fed into the vacuum chamber held in a N₂ atmospherewith the degree of vacuum 5×10⁻² torr.

Vacuum deposition in the order of Zn→Mg→Zn was performed in this vacuumchamber. In this example, an adhesion amount in total was adjusted to 30g/m² per single face by controlling first Zn deposition at 17 g/m², Mgdeposition at 1 g/m² and secondary Zn deposition at 12 g/m². Thetemperature of the parent sheet before the vapor deposition wascontrolled such that the temperature of the steel sheet after thecompletion of vapor deposition was within the range of 270°-370° C. Theheat retained in the steel sheet effectively promoted mutual diffusionbetween Zn and Mg. In the case of forming the coating layer having thepenta-layered structure shown in FIG. 4, the steel sheet after thecompletion of vapor deposition was controlled at 345°-370° C.

A Zn-Fe or Zn-Fe-Mg alloy layer of 0.01-0.1 μm in thickness was formedby the temperature control, and a Zn-Mg binary coating layer having thetri-layered structure (shown in FIG. 3) or the penta-layered structure(shown in FIG. 4) was formed on the Zn-Fe or Zn-Fe-Mg alloy layer.

The second sublayer in FIG. 3 and the third sublayer in FIG. 4 had Mgconcentration of approximately 12 wt. % and the mixed structure of a Zn₂Mg phase with a Mg-dissolved Zn phase as shown in FIG. 5. The topsublayer (i.e. the third sublayer in FIG. 3 or the fifth sublayer inFIG. 4) and the first sublayer (in FIG. 3 or 4) had Mg concentration ofapproximately 0.1 wt. %. The second and forth sublayers in FIG. 4 had Mgconcentration of approximately 5 wt. % and the mixed structure of a Zn₁₁Mg₂ phase with a Mg-dissolved Zn phase.

The coating layer having the tri-layered structure shown in FIG. 3comprised the first sublayer of 2.5 μm in thickness, the second sublayerof 0.8 μm in thickness and the third sublayer of 1 μm in thickness. Thecoating layer having the penta-layered structure shown in FIG. 4comprised the first sublayer of 2.3 μm in thickness, the second sublayerof 0.2 μm in thickness, the third sublayer of 1 μm in thickness, theforth sublayer of 0.2 μm in thickness and the fifth sublayer of 0.8 μmin thickness.

In addition, after vapor deposition was performed under the conditionthat the temperature of the steel sheet was at 230° C. at the completionof the vapor deposition, the steel sheet was heated 5 hrs. at 150°-240°C. in a N₂ atmosphere to accelerate mutual diffusion between Zn and Mg.The Zn-Mg binary coating layer having the tri-layered structure (FIG. 3)or the penta-layered structure (FIG. 4) was formed by said heattreatment, too. In this case, the heating temperature was predeterminedwithin the range of 160°-180° C. for the tri-layered structure or withinthe range of 200°-220° C. for the penta-layered structure.

Each coated steel sheet obtained in this way was covered with a Mg-richlayer. Since the Mg-segregated layer would cause blackening, it wasremoved by a 0.5%-HCl solution.

Research for Corrosion Resistance

A test piece having the dimensions of 100 mm×200 mm was cut out from theeach steel sheet coated with the Zn-Mg binary coating layer and offeredto the complexed cycle corrosion test shown in FIG. 7. This corrosiontest imitates an outdoor corrosive atmosphere and is likely to form rustat cut end faces.

As for a comparative example 5, a steel sheet coated with a Zn-Mg alloylayer in the same adhesion amount of 30 g/m² as that in Example 3 shownin FIG. 3 or 4 was manufactured by the vapor deposition in the order ofZn deposition→simultaneous Zn and Mg deposition→Zn deposition. In thiscase, the Mg deposition was adjusted to the same amount of 1 g/m² asthat in Example 3. The relation of each sublayer in thickness and theconcentration were controlled at the same levels as those in Example 3,too. The middle sublayer in the coating layer obtained in this way hadthe mixed structure of a Zn₂ Mg phase with a Zn₁₁ Mg₂ phase, but thepresence of a Mg-dissolved Zn phase was not detected.

A vapor Zn-coated steel sheet in an adhesion amount of 30 g/m² wasmanufatured as a comparative example 6.

A test piece cut out from each coated steel sheet was examined by thecorrosion test to research corrosion resistance at a cut end face. Theresults are shown in Table 8. It is noted from Table 8 that any steelsheet coated with the Zn-Mg binary coating layer according to thepresent invention endured over a long time until the formation of rustat the cut end face as compared with the comparative example 5. The longendurance time means the excellent corrosion resistance of the coatedsteel sheet according to the present invention. The excellent corrosionresistance is supposed to be derived from the formation of the Zn-12% Mgsublayer having the mixed structure of the Zn₂ Mg phase with theMg-dissolved Zn phase. On the contrary, in the case of the test piece ofthe comparative Example 5 coated with a coating layer having a Zn-12% Mgintermediate sublayer composed of the mixed structure of the Zn₂ Mgphase with the Zn₁₁ Mg₂ phase, rust was detected at the cut end faceafter 13 cycles of the corrosion test at the longest.

The coated steel sheet according to the present invention had a Zn-Fe orZn-Fe-Mg alloy layer formed at the boundary between the substrate steeland the Zn-Mg binary coating layer effective in adhesiveness. Forinstance, the peeling of the coating layer was not detected at all, bythe test wherein the coated steel sheet was bent with 0t (i.e. thecoated sheet was bent with the angle of 180 degrees without theinsertion of any sheet), an adhesive tape was applied onto the bent partand then the adhesive tape was peeled off the surface.

                                      TABLE 8                                     __________________________________________________________________________    CORROSION RESISTANCE OF COATED STEEL SHEET AT CUT END FACE                                                 STRUCTURE           REPETITION                                                OF INTER-   LAMELLAR                                                                              CYCLE NOS.                                                MEDIATE LAYER                                                                             STRUCTURE                                                                             UNTIL RUST                             STEP OF            SECOND LAYER IN                                                                           OF      FORMATION                    KIND OF   VAPOR    DIFFUSION OF                                                                            FIG. 3 AND THIRD                                                                          COATING AT CUT END                   COATING LAYER                                                                           DEPOSITION                                                                             Zn AND Mg LAYER IN FIG. 4                                                                           LAYER   FACE   NOTE                  __________________________________________________________________________    Zn--Mg alloy                                                                            Zn deposition                                                                          diffusion                                                                             mixed structure of                                                                          FIG. 3  25     examples of                        ↓                                                                            by heat Zn.sub.2 Mg phase with Mg    the present                     Mg deposition                                                                          retained in                                                                           dissolved Zn phase                                                                          FIG. 4  45     invention                          ↓                                                                            steel sheet                                                          Zn deposition                                                                          heating mixed structure of                                                                          FIG. 3  25                                              5 hrs. in N.sub.2                                                                     Zn.sub.2 Mg phase with Mg                                             atmosphere                                                                            dissolved Zn phase                                                                          FIG. 4  45                                     Zn deposition →                                                                 --      mixed structure of                                                                          FIG. 3  10     comparative                     simultaneous                                  example 5                       Zn and Mg        Zn.sub.3 Mg phase with                                                                      FIG. 4  13                                     deposition →                                                           Zn deposition    Zn.sub.11 Mg.sub.2 phase                           Zn        Zn deposition                                                                          --      --            single   5     comparative                                                    Zn phase       example               __________________________________________________________________________                                                            6                 

Example 4

Steps of Manufacturing of Coated Steel Sheet

An unannealed cold-rolled steel sheet of 1.0 mm in thickness and 918 mmin width having the composition of 0.023 wt. % C, 0.24 wt. % Si, 0.24wt. % Mn, 0.013 wt. % P, 0.007 wt. % S, 0.019 wt. % Al and the balanceFe was used as a parent sheet. This parent sheet was processed in theplant whose layout is shown in FIG. 6, to manufacture a steel sheetcoated with a vapor Zn-Mg binary layer.

A duct 20 was held in a N₂ atmosphere having O₂ concentration of 0.001vol. % and H₂ O concentration of 0.06 vol. %. The time period of theparent sheet 1 passing through the duct 20 was predetermined at 70seconds. The temperature of the parent sheet 1 was controlled so thatthe steel sheet was held at a predetermined temperature when enteringinto each of the primary Zn deposition compartment 40 and the first Zndeposition compartment 60. In this case, since the steel sheet wasslightly heated by vapor deposition, the temperature of the steel sheetwas adjusted lower by 10° C. or less compared with the predeterminedvalue in response to the rising of the temperature during the vapordeposition. The deposition amounts in the first Zn depositioncompartment 60 and the second Zn deposition compartment 70 werepredetermined at 10 g/m² per single face. The steel sheet 4 after thevapor deposition was carried into a high-frequency heating furnace 80and then heated 5 seconds at 310° C. in a N₂ atmosphere.

Research for Effects of Primary Zn Deposition, Mg Deposition andTemperature of Steel Sheet on Adhesiveness and Structure

The amount of primary Zn deposition, the amount of Mg deposition and thetemperature of the steel sheet during vapor deposition were variouslychanged to research those effects on the adhesiveness and structure of aformed coating layer. The adhesiveness was examined by the samebending-peeling test as that in Example 4, and evaluated as good whenthe peeling of the coating layer was not detected at all. The structureof the coating layer was testified by observing the section of thecoating layer with SEM, and its density was researched. The results areshown in Table 9.

It is noted from Table 9 that a Zn-Mg binary layer having densestructure excellent in adhesiveness was formed, when primary Zn wasdeposited on the steel sheet held at 180° C. or higher with a depositionamount equal to 1.5 times that of Mg deposition.

On the contrary, the partial or wholly peeling of the coating layer wasdetected in the coated steel sheet when the deposition ratio of primaryZn to Mg was less than 1.5. That is, the lower deposition ratio causespoor adhesiveness. The coating layer was partially peeled off, when thecoated steel sheet was manufactured with the adhesion ratio below 1.5without the heat treatment. When the temperature of the steel sheet wasbelow 180° C. during vapor deposition, vacancies were observed in theformed coating layer, although the coating layer was good ofadhesiveness. These results means that the adhesion ratio of primary Znto Mg and the temperature of the steel sheet played important roles inthe improvement of adhesiveness and dense structure.

                                      TABLE 9                                     __________________________________________________________________________    EFFECTS OF PRIMARY Zn DEPOSITION AND TEMPERATURE OF                           STEEL SHEET ON ADHESIVENESS AND STRUCTURE OF COATING LAYER                    AMOUNT OF                                                                            AMOUNT OF                                                                            DEPOSITION                                                                           TEMP. OF STEEL                                           PRIMARY Zn                                                                           Mg     RATIO OF                                                                             SHEET DURING                                                                             EVALUTATION OF                                DEPOSITION                                                                           DEPOSITION                                                                           PRIMARY Zn                                                                           VAPOR DEPOSITION                                                                         COATING LAYER                                 (g/m.sup.2)                                                                          (g/m.sup.2)                                                                          TO Mg  (°C.)                                                                             ADHESIVENESS                                                                           STRUCTURE                                                                            NOTE                          __________________________________________________________________________    0.9    0.5    1.8    220        good     dense  examples                      2.2    0.5    4.4    220        good     dense  of the                        5.7    0.5    11.4   220        good     dense  present                       1.8    1.2    1.5    220        good     dense  inven-                        2.9    1.2    2.4    220        good     dense  tion                          5.2    1.2    4.3    220        good     dense                                8.3    1.2    6.9    220        good     dense                                2.2    0.5    4.4    180        good     dense                                8.3    1.2    6.9    180        good     dense                                0.8    1.2    0.7    220        peeled*.sup.1                                                                          dense  compara-                      1.5    1.2    1.3    220        peeled*.sup.2                                                                          dense  tive                          2.2    0.5    4.4    150        good     vacancies                                                                            examples                      8.3    1.2    6.9    160        good     vacancies                            __________________________________________________________________________     NOTE 1: The coating layer was completely peeled off.                          NOTE 2: The coating layer was partially peeled off.                      

Example 5

O₂ concentration X (vol. %) and H₂ O concentration Y (vol. %) of anatmosphere in the duct 20 were variously changed together with a timeperiod (seconds) for passing a steel sheet though the duct 20 and anamount of H₂ added to the atmosphere, to research those effects on theadhesiveness of a formed coating layer. In this example, a depositionamount of primary Zn was predetermined at 5 g/m², a deposition amount ofMg was predetermined at 1.2 g/m², and the temperature of the steel sheetduring vapor deposition was adjusted to 250° C. The steel sheet 4 afterthe vapor deposition was heat treated at 300° C. in the high-frequencyheating furnace 80 held in a N₂ atmosphere.

The results are shown in Tables 10 and 11. It is noted from Table 10that a coating layer was formed with good adhesiveness under theconditions satisfying the relationships of X×Z≦1.2 and Y×Z≦35. On theother hand, the adhesiveness was inferior, if any one of therelationships of X×Z≦1.2 and Y×Z≦35 was not satisfied, as shown in Table11.

In the case when H₂ was added to the atmosphere in the duct, goodadhesiveness was obtained under the conditions of X×Z≦3.8 and Y×Z≦80. Onthe other hand, the adhesiveness was inferior, if any one of therelationships of X×Z≦3.8 and Y×Z≦80 was not satisfied. The effect of H₂to relax the condition of the atmosphere was apparent, when H₂ was addedin an amount of H₂ was 0.05 vol. % or more.

                  TABLE 10                                                        ______________________________________                                        EFFECTS OF O.sub.2 AND H.sub.2 O CONCENTRATION OF ATMOSPHERE                  IN DUCT AND PASS TIME OF STEEL SHEET ON                                       ADHESIVENESS OF COATING LAYER (PRESENT INVENTION)                                                 ADDITION                                                  X × Z                                                                            Y × Z                                                                              AMOUNT OF  ADHESIVENESS OF                                (vol. % · sec.)                                                               (vol. % · sec.)                                                                 H.sub.2 (vol. %)                                                                         COATING LAYER                                  ______________________________________                                        0.01     5.3        0          good                                           0.07     5.0        0          good                                           0.2      5.5        0          good                                           0.5      5.5        0          good                                           1.2      5.2        0          good                                           1.0      8.5        0          good                                           0.5      15.8       0          good                                           1.2      25.3       0          good                                           1.2      34.1       0          good                                           1.5      25.3       0.1        good                                           3.2      25.3       0.5        good                                           3.2      55.0       1.5        good                                           3.2      78.9       1.5        good                                           3.8      78.9       4.0        good                                           ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        EFFECTS OF O.sub.2 AND H.sub.2 O CONCENTRATION OF ATMOSPHERE                  IN DUCT AND PASS TIME OF STEEL SHEET ON ADHESIVE-                             NESS OF COATING LAYER (COMPARATIVE EXAMPLE)                                                       ADDITION                                                  X × Z                                                                            Y × Z                                                                              AMOUNT OF  ADHESIVENESS OF                                (vol. % · sec.)                                                               (vol. % · sec.)                                                                 H.sub.2 (vol. %)                                                                         COATING LAYER                                  ______________________________________                                        1.6      8.5        0          completely peeled                              1.0      39.2       0          completely peeled                              1.6      37.2       0          completely peeled                              4.3      25.3       0.5        partially peeled                               1.5      85.4       1.5        partially peeled                               4.3      85.4       4.0        completely peeled                              3.2      25.3       0.01       completely peeled                              ______________________________________                                    

According to the present invention as afore-mentioned, the formed Zn-Mgbinary coating layer has the structure that a ductile low-Mg sublayer isinterposed between substrate steel and a high-Mg sublayer. Due to thisspecified structure, the high-Mg sublayer effectively protects thecoated steel sheet against corrosion, while the low-Mg sublayereffectively absorbs the difference in deformation between the hardhigh-Mg sublayer and the substrate steel during working the coated steelsheet.

The harmful influence of the high-Mg sublayer on water-proof secondarypaint adhesiveness is suppressed by forming the top sublayer composed ofa Zn-Mg alloy having Mg concentration of 0.5 wt. % or less with anadhesion amount of 0.3 g/m² or more. Regardless the formation of the toplow-Mg sublayer, the intrinsic properties of the high-Mg sublayer arewell exhibited to assure superior corrosion resistance.

Consequently, the coated steel sheet according to the present inventionis excellent in workability as well as corrosion resistance due to thehigh-Mg sublayer. In addition, since Mg concentration on the surface islowered, the coated steel sheet is improved in spot weldability, too.The coated steel sheet has the feature that secondary paint adhesivenessis well balanced with the high corrosion resistance, so that it isuseful as structural members or parts exposed to a severe corrosiveatmosphere in various industrial fields.

The adhesiveness of the coating layer is improved by vapor depositingprimary Zn before vapor Mg deposition with the adhesion ratio of 1.5times Mg deposition or more, when the coated steel sheet is manufacturedby vapor deposition process. The primary Zn deposition is effective forinhibiting a brittle intermediate layer at the boundary between thesubstrate steel and the coating layer, too. The adhesiveness of thecoating layer is also improved by controlling the composition of anatmosphere leading to a vacuum chamber so as to inhibit the re-oxidationof the steel surface activated by reductive heating.

What is claimed is:
 1. A method of manufacturing a steel sheet coatedwith a Zn-Mg layer having one of a tri-layered or penta-layeredstructure comprising a first sublayer including a Zn-Mg alloy having aMg concentration of about 0.5 wt. % or less, an intermediate sublayerincluding a Zn-Mg alloy having a Mg concentration of about 7 wt. % ormore and an outermost sublayer including a Zn-Mg alloy having a Mgconcentration of about 0.5 wt. % or less, wherein the sublayers aresuccessively laminated on a substrate steel wherein a Zn-Fe or Zn-Fe-Mgalloy layer is formed at a boundary between the substrate steel and theZn-Mg coating layer, said method comprising the steps of:vapordepositing primary Zn on a surface of a steel sheet; and successivelyvapor depositing Mg and Zn on said surface,wherein a ratio of saidprimary Zn deposition to said Mg deposition is controlled to a value ofabout 1.5 or more.
 2. The manufacturing method according to claim 1,wherein the steel sheet is held at a temperature of about 180° C. orhigher during vapor deposition.
 3. A method of manufacturing a steelsheet coated with a Zn-Mg Layer, having one of a tri-layered orpenta-layered structure comprising a first sublayer including a Zn-Mgalloy having a Mg concentration of about 0.5 wt. % or less, anintermediate sublayer including a Zn-Mg alloy having a Mg concentrationof about 7 wt. % or more and an outermost sublayer including a Zn-Mgalloy having a Mg concentration of about 0.5 wt. % or less, wherein thesublayers are successively laminated on a substrate steel wherein aZn-Fe or Zn-Fe-Mg alloy layer is formed at a boundary between thesubstrate steel and the Zn-Mg coating layer, said method comprising thesteps of:carrying a steel sheet having a surface activated by reductiveheating through a reducing atmosphere, a chamber held in a N₂atmosphere, a duct held in a N₂ atmosphere and inlet sealing rolls intoa vacuum chamber; and successively vapor depositing primary Zn, Mg andZn on the surface of said steel sheet, wherein said steel sheet ispassed through said duct under conditions satisfying the relationshipsof X×Z≧1.2 and Y×Z≅35, wherein X represents a vol. % O₂ concentration ofthe atmosphere in said duct, Y represents a vol. % H₂ O concentration ofthe atmosphere in said duct, and Z represents the time period of saidsteel sheet passing through said duct.
 4. The manufacturing methoddefined in claim 3, wherein H₂ is added to the atmosphere in the duct.5. The manufacturing method defined in claim 4, wherein the steel sheetis passed through the duct held in the atmosphere to which H₂ is addedin an amount of 0.05-4 vol. % under the conditions satisfying therelationships of X×Z≦3.8 and Y×Z≦80.
 6. A method of manufacturing asteel sheet coated with a Zn-Mg layer having one of a tri-layered orpenta-layered structure comprising a first sublayer including a Zn-Mgalloy having a Mg concentration of about 0.5 wt. % or less, anintermediate sublayer including a Zn-Mg alloy having a Mg concentrationof about 7 wt. % or more and an outermost sublayer including a Zn-Mgalloy having a Mg concentration of about 0.5 wt. % or less, wherein thesublayers are successively laminated on a substrate steel wherein aZn-Fe or Zn-Fe-Mg alloy layer is formed at a boundary between thesubstrate steel and the Zn-Mg coating layer, said method comprising thesteps of:successively vapor depositing Zn, Mg and then Zn on a surfaceof a steel sheet; and controlling the temperature of said steel sheetwithin the range of about 270°-370° C. at a completion of vapordeposition, whereby the coating layer is conditioned to one of thetri-layered or penta-layered structure by the mutual diffusion betweenMg and Zn.
 7. A method of manufacturing a steel sheet coated with aZn-Mg layer having one of a tri-layered or penta-layered structurecomprising a first sublayer including a Zn-Mg alloy having a Mgconcentration of about 0.5 wt. % or less, an intermediate sublayerincluding a Zn-Mg alloy having a Mg concentration of about 7 wt. % ormore and an outermost sublayer including a Zn-Mg alloy having a Mgconcentration of about 0.5 wt. % or less, wherein the sublayers aresuccessively laminated on a substrate steel wherein a Zn-Fe or Zn-Fe-Mgalloy layer is formed at a boundary between the substrate steel and theZn-Mg coating layer, said method comprising the steps of:successivelyvapor depositing Zn, Mg and then Zn on a surface of a steel sheet; andheating said steel sheet about 1 hr. or longer at about 150°-240° C.after the completion of vapor deposition, whereby the coating layer isconditioned to one of the tri-layered or penta-layered structure by themutual diffusion between Mg and Zn.